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... and does not create cold rivers and lakes. Thus we don't find the parallel evolution of fish into similar forms in the southern hemisphere. The Salmonids can not in any manner cross the warm tropical region and therefore did not colonize what suitable regions there are for them in the southern hemisphere. But with man's help they can, and rainbow trout are now found on every continent except Antarctica.
Let us assume that an original Salmonid ancestor originated in a cold fresh water lake or stream. This new species would have two main methods of dispersal. First, it would be natural for this Salmonid to begin foraging down river into the estuary and ocean where food is much more abundant. This adaptation to anagram would be highly advantageous because the fish would be getting a wealth of food from the ocean but retain the safety of fresh water spawning where there are less predators. It would then be only a matter of time until group of fish strayed off course into a new river up or down the coast. In this manner, Salmonids could colonize every river in the northern hemisphere where the water was cold enough.
The second method of dispersal is made possible by glaciation. As the ice sheets expand and contract from North to South across the face of Eurasia and North America, immense amounts of meltwater form huge lakes on their southern borders. These lakes allowed Salmonids to jump between river drainage systems without ever entering salt water. Pielou talks about an excellent example of this called Lake Agassiz. This huge lake, once at least four times as large as Lake Superior, marked the southern edge of the Laurentide ice sheet from 12 to 7. 5 thousand years ago. When the ice sheet was at its greatest extent, this lake drained out the Mississippi basin into the Gulf of Mexico.
Freshwater fish from the Mississippi system colonized the lake. As the glacier retreated to the North, Lake Agassiz began draining out to the Atlantic via the St. Lawrence River, and fish species from the lake moved down into the river as others from the St. Lawrence moved up into the lake. This process happened many times in Lake Agassiz and other pro glacial lakes in North America and Eurasia dispersing freshwater Salmonids. In fact, this phenomenon has even allowed dispersal across the continental divide of North America where there are North South valleys due to folding of the earth's crust.
Man and the Future of the Salmoninae We have seen how the Salmoninae have coped with forces of nature in the past. But in the past few thousand years, a new force unlike any they have ever faced has evolved: man. Homo sapiens is an organism that alters its environment like no other, and his actions have enormous effects on the earth and other organisms. Man must perceive the decisions he makes as things that simultaneously have positive and negative consequences for himself.
He should strive to make decisions with the maximum positive effect. Unfortunately for certain Salmoninae, many actions that are overall positives for man, are huge negatives for fish. The cutting of forests gives man useful materials but disturbs rivers with log jams and erosion. Dams for hydroelectric power and irrigation are immense positives for man, but block the migration of anadromous fish.
The San Joaquin salmon, the world's most southerly run, is now extinct for this reason. Industry gives immense wealth to our economy, but its wastes ruin the life of rivers. If the wastes are poisonous like insecticides from agriculture, they can accumulate in the bodies of fish until they are lethal, like DDT. If the wastes are organic, they may be broken down by bacteria effectively de oxygenating a river in the process.
I could go on and on about how man, in benefiting himself, is turning rivers into the urethra's of modern industrial society. We can see that man's actions are killing many rivers, and the group of Salmoninae that are most adversely affected are the anadromous ones because they use the rivers as migration routes. But what happens when man's desires fall in line with other organisms? Many people do love to fish, and man is capable of keeping special areas relatively free from pollution. This has led to a major expansion of the range of trout.
Although there are surely less salmon around today than any time in the past, trout, especially the rainbow (native to Northern California) are far more widespread. Dams on the Colorado release cold, high-pressure water from the depths of the lakes they create to spin turbines for electricity. This has cooled the river and made it much more hospitable for trout. And since they do not need to migrate, trout now flourish there. The downside to this is that it decreases biodiversity, the truly important resource for man. It is normally the rainbow trout that is introduced everywhere which may out compete or hybridize with native species of fish.
The relatively small salmon run of the San Joaquin was a small price to pay for the world's most productive agricultural region. But in other regions, man has an interest in keeping the salmon alive. In the Pacific Northwest, many salmon hatcheries have been built to enhance their runs. The government does studies that analyze whether hatcheries are a worthwhile decision. In 1981 for the coho salmon, the benefit to cost ratio was 7 to 1. As long as the species benefits man, it will survive.
Conclusion Man always makes the decision that appears to benefit him the most. In the past, many of these decisions have adversely affected organisms around him, the Salmoninae occasionally being one of them. But do we really know what is best for us in the long run? All that we can do is base our decisions on the knowledge we do have and hope for the best. So where can we turn to for the knowledge upon which to base our decisions? The answer is to study the past.
Interpreting the evidence of how organisms have survived and gone extinct and why will give us the keys to survival. Recently, study of the evolution of life has changed man's perspective on what is important. We have learned that diversity is a key for survival, and this has immediate relevance to our decisions today. Man will be less likely to terminate a species, because its extinction will actually be seen as a negative for him as a loss of diversity.
Thus man's interests are in effect falling in line with the organisms around him, including the Salmoninae. Bibliography Frost & Brown, The Trout, Collins, London, 1967. Mills, Salmon & Trout, Oliver & Boyd, Edinburgh, 1971. Pielou, After the Ice Age, The University of Chicago Press, Chicago, 1991.
State of California-Resources Agency, Department of Fish and Game, An Assessment of Federal Water Projects Adversely Affecting California's Salmon and Steelhead Resources. U. S. Department of Commerce, Columbia River Fisheries Development Program, January 1981.
Vladikov, Vadim V. , A Review of Salmonid Genera and Their Broad Geographical Distribution, Transactions of the Royal Society of Canada, vol. 2, series IV, June, 1963, sec. III. Wales, W. B. , Trout Biology, University of Wisconsin Press, London England, 1981. [i]Illustrations (1) Page 1, Columbia River Fisheries Development Program. (2) Page 16, Trout Biology. (3) & (4) A Review of Salmonid Genera and Their Broad Geographical Distribution. (5) Page 12, Columbia River Fisheries Development Program. [/i]
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Research essay sample on Southern Hemisphere Columbia River
